1. Field of the Invention
The present invention relates to a communication system suitable for use in a multiplexing apparatus or the like, and specifically to a system for carrying out data communication between respective communication cards mounted in shelf configurations for controlling the transfer of data between the respective communication cards.
2. Description of the Related Art
FIG. 5 is a view showing the structure of a system for effecting data communication between respective communication cards using a conventional paired-address method. In FIG. 5, reference numeral 5 indicates an upper shelf which accommodates therein one or more trunk cards (also hereinafter called "cards A") 2 and an exchange control card 100 for controlling data communication between respective communication cards. Reference numeral 6 indicates a lower shelf which accommodates one or more trunk cards (also hereinafter called "cards B") 4 therein. Further, designated by numeral 10 is a communication card selection bus for causing a communication card selection signal to pass therethrough, and designated by numeral 11 is a bidirectional data bus.
The operation of the above communication system will now be described below. When it is necessary to effect the transfer of data between the trunk cards 2 and 4, the exchange control card 100 sends addresses of both a communication card A and a communication card B to the communication card selection bus 10 as shown in FIG. 6.
Accordingly, the communication card A, which has been selected during the first period (t.sub.1) of the entire period corresponding to a pair of addresses, brings data "dB" from the data bus 11 in synchronism with a clock (t.sub.3) after two clocks. Further, the communication card A outputs data "dA" to the data bus 11 in synchronism with a clock (t.sub.4) subsequent to three clocks. On the other hand, the communication card B, which has been selected during the latter half (t.sub.2) of the entire period corresponding to the pair of addresses, outputs data "dB" to the data bus 11 in synchronism with the clock (t.sub.3) after one clock and brings data "dA" from the data bus 11 in synchronism with the clock (t.sub.4) after the two clocks.
Thus, the transfer of the data between the respective communication cards is carried out by using the pair of addresses as a single data transfer unit.
The conventional system for effecting the data communication between the communication cards mounted in the shelf configurations are constructed as described above. Therefore, the conventional system is accompanied by problems that when the transfer of the data between the communication cards which belong to the shelves 5, 6 different from each other, is carried out, the amount of delay of data with respect to the other increases, thereby causing inconvenience that data on the data bus 11 is not yet determined in data take-in timing, and the transfer of the data between the respective communication cards cannot be securely effected when a data transfer speed increases.
Incidentally, as the prior arts, there are known techniques disclosed in Japanese Patent Application Laid-Open Publication Nos. 623-155694 and 62-161295.